Laser line and broadband beamsplitters are in wide use in optical systems, with cube beamsplitters being more common. Cube beamsplitters are generally made from pairs of triangular glass prisms that have been bonded together. Bonding may be carried out using several different methods: optical adhesive bonding, optical direct bonding, and diffusion bonding.
With optical adhesive bonding, the thickness of the adhesive can be adjusted to reflect half the light of a given wavelength incident on one face of the cube and transmit the other half; however, this technique is not without drawbacks. Optical adhesive bonding bonds two polished mirror surfaces with optical adhesive such as, for example, polyester, epoxy, or urethane-based adhesive. Optical adhesive bonding is simple and can be used to bond different materials together; however, the presence of adhesive may lead to flaws or discolorations of the surface. Additionally, it is difficult to match the refractive index of the adhesive material to that of the optical components. Light from adhesive bonded devices loses flux through Fresnel reflection. Finally, the adhesive can be deformed, softened, or degraded when used in a laser system.
Optical direct bonding occurs when two ultrasmooth surfaces are held in close contact without any adhesive. It is thought that smooth surfaces in close enough contact will be electromagnetically attracted to one another. Under the right conditions, this type of bonding can be stronger than optical adhesive bonding. However, optical direct bonding is generally only suited to bonding between two prisms of the same material, since inconsistent expansion will occur due to differences in thermal expansion coefficients when the optical devices are heated (such as, for example, upon exposure to a laser). Inconsistent expansion can lead to separation of components and is particularly common in high-powered laser systems.
Meanwhile, high-temperature diffuse bonding is similar to optical direct bonding except that contact is followed by high-temperature heat treatment so that diffusion of atoms from one interface to the other can occur. However, as with optical direct bonding, high-temperature diffuse bonding is best suited to bonding crystal materials of the same type.
Cube beamsplitters currently on the market can split wavelengths of light down to about 250 nm. Challenges in the shorter wavelength regions include the lack of existence of absorption-free adhesives as well as the porous nature of fluoride coatings currently in use, especially for the ArF laser beam line at 193 nm. Chemical substances used as adhesives or during the bonding process can be absorbed by porous layers, leading to high absorption at short wavelengths. Due to these challenges, hybrid optical devices constructed from fused silicas such as HPFS® (Corning, Inc.) and CaF2 are not currently available. However, such optical devices would be useful as ArF laser cube beamsplitters, as well as in other applications.
In the short wavelength region, standard optical bonding processes such as epoxy bonding, frit bonding, diffusion bonding, and optical contacting thus cannot be used due to absorption. One state-of-the-art bonding technology, chemically activated direct bonding (CADB), provides an epoxy-free solution assisted optical-contacting process. However, this process requires chemical soaking and thermal annealing, eliminating its applications for thermally sensitive crystal materials such as CaF2.
Typically, excimer lasers of low wavelengths must be operated at power levels lower than their maximum or, alternatively, users of these lasers had to accept shorter device lifetimes while operating at lower power levels, due to degradation of beamsplitters. Uncoated CaF2 surfaces, for example, degrade after only a few million pulses when subjected to pulse energies above 40 mJ/cm2 using 193 nm excimer radiation. Although ArF excimer lasers typically operate at lower pulse energies, local non-uniformities in the beam may be higher than the average value and thus exceed the damage threshold.
What is needed is an absorption free bonding method for constructing optical devices such as beamsplitters. Ideally, this method could be used to construct optical devices exhibiting no scatter loss or absorption loss such as commonly seen with optical adhesive bonding and could be used with thermally sensitive materials and porous coatings. Further, devices constructed by this method would be more durable than currently available technology. The present invention addresses these needs.